The experiments described in this proposal are directed at gaining a more complete understanding of the cell biology and molecular phenotype of the mammalian liver stem cell in the developing embryo and fetus. This will be accomplished by: (l) the derivation of continuous cells lines from the cells of the prehaptic and hepatic endoderm of the mouse embryo and fetus on embryonic days e 8.5 to e 9.5, (2) the study of gene expression in these continuous cell lines and in the mouse embryo and fetus to determine if the cell lines and their differentiated derivatives are similar in their gene expression to the cells of the hepatic lineage in the mouse embryo and (3) examining the capacity of the prehaptic and hepatic cell lines to differentiate in vitro and in vivo. This work will provide the basis for the development of an in vitro system which will be useful in the study of events that are important in the ontogenesis of the mammalian liver. In addition, a more complete understanding of the phenotype of liver stem cells in the embryo will provide a comparison for putative stem cells in the adult liver.